The Tyrant is big and mostly slow. Easy target to hit. It also has a pretty long primary attack range (for a melee attack).
There are several ways to kill big aliens that attack your base, one of them is to slip behind them while they are attacking turrets with a Painsaw or another weapon that does a lot of damage, they get stuck and can't backpedal, in this moment team support is vital. You can probably take down an unexperienced alien player on your own, but the good players notice how their health goes down quickly, your teams job is to kill him before he kills you. If the tyrant gets to pass by the blocking player, he must be chased outside by 3-4 marines (at least 2 of em with decent armour) Lasgun or Mass Driver are excellent weapons to kill big aliens on the run, Pulse/L.Cannon is good if they're attacking, but to chase its better to use hitscan weapons. Always expect to find more resistance along the way, I can't count how many times I've chased a dying Tyrant or Dragoon and end up getting killed by another alien, this is why team support is very important.
The best 'strategy' for stale end games is to starve the other team at all costs, this means to kill the better players without feeding them any kills, its quite a challenge when its a tyrant, but it can be done. An alien player can have 9 evolution points, and he looses 5 to evolve to tyrant, if he doesn't get any kills and dies, he'll have to go Adv Dragoon and get 2 kills or more, or stay dretch and get 1 kill. Note that not feeding doesn't mean sit and hug a turret... You
have to chase the dying aliens. If you don't have enough credits to at least buy armour+helmet you can always grab a ckit and help mantain the defenses.
People also have to understand that going out with only light armour is one of the worst possible ideas unless you are chasing, and even then you'd need support from people with better armour and weapons. A Tyrant can kill light armour on a single slash, so to handle 5 humans in light armour isn't really a problem for the better alien players.
After killing a few big aliens you'll start noticing that the players using a Tyrant a while ago are now dying as dretches, at this moment your team ussually have enough credits to pull off a good attack. (check out in the
Strategy Guide the Base Raid post by juice).
Note that this strategy applies more or less to aliens as well. After Humans get flamers its better to stay off them untill you have a Marauder or better, also, if you have a small class its better to stick to walls and get away from the big aliens, since you can get them killed by blocking them, if theres a Bsuit and a Dragoon trying to attack him while you're a dretch, stay away and try not to block the Dragoons pounce.
Starving the other team pretty much works for both teams.